The present invention relates generally to computed tomography systems, and more particularly, to image reconstruction when performing a scan with a tilted gantry using a multislice computed tomography system.
In a typical CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane." The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element or cell of the array produces a separate electrical signal that is a measurement of the beam attenuation at that detector location. The attenuation measurements from all the detector cells are acquired separately to produce a transmission profile.
In typical CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the rotational angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry rotational angle is referred to as a "view." A "scan" of the object comprises a set of views made at different gantry rotational angles, or view angles, during one revolution of the x-ray source and detector. In a scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. This process is called image reconstruction, and there are many image reconstruction techniques. One such method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units," which are used to control the brightness of a corresponding pixel on a cathode ray tube display and hence produce the image.
Image slices typically are acquired so that each slice is perpendicular to a longitudinal axis of the patient, i.e., the slices are substantially parallel to each other and spaced in the z direction. It often is preferable to acquire slices at a non-perpendicular orientation, however, to better visualize certain structures within the patient. For example, slices at angles other than 90 degrees to the longitudinal axis of the patient, i.e., non-transverse slices, are preferable when studying certain internal organs.
To provide such an angular orientation, the gantry of the system is tilted so that the axis of rotation of the x-ray source, or the gantry axis, is inclined relative to the axis of the patient. The gantry is tilted about a pivot point so that the gantry may be positioned at different tilt angles. An encoder or some other transducer is coupled to the gantry to detect the tilt angle. The angular orientation (theta) of the gantry about the pivot point, as indicated by the encoder or transducer, is supplied to a control processor that is programmed to control operation of the system.
Utilizing a tilted gantry to obtain images is problematic, however, in a CT system having multiple rows of detectors in the z-direction, known as a "multislice" system. The projection data acquired by a multislice system comprises data having different centers of rotation, due to the multiple rows of detectors in the z-direction. Typical image reconstruction methods, however, assume the data has the same center of rotation. Upon reconstruction of the image from the projection data using current systems, the different centers of rotation cause the image center to be shifted up or down.
Further, for axial CT scans, also known as stop-and-shoot scans, further error is introduced into a multislice system by the current table indexing methods. Present systems advance the table for each slice by the thickness of the x-ray beam in the z-direction. With a tilted gantry and multiple rows of detectors, however, this table indexing results in uneven slice spacing.
Additionally, current image reconstruction techniques create problems for helical CT scans using a tilted gantry. In a helical CT scan, the table constantly moves the patient in the z-direction at a specified speed while the data for the prescribed number of slices is acquired. Such a scan generates a single helix from a one fan beam helical scan, or in the case of multislice systems, multiple helixes. The helix mapped out by a fan beam yields projection data from which images in each prescribed slice may be reconstructed. In a multislice system, portions of projection data from multiple fan beams may be utilized to reconstruct a single slice. The differing centers of rotation for each portion of the data used to reconstruct the image cause artifacts and other image errors for a multislice system. Further, some current multislice systems combine projection data prior to the filtered back projection operation, mentioned above, to increase reconstruction speed. Combining projection data having differing centers of rotation, however, causes artifacts and other image errors in the reconstructed image.
Thus, utilizing current reconstruction techniques with a multislice system having a tilted gantry results in artifacts and other unacceptable defects in the image. Yet, implementing special, new reconstruction techniques only for tilted gantry scans in a multislice system would be time consuming, expensive and would create exceptions to the current standard techniques. Therefore, it is desirable to maintain the current reconstruction techniques and integrate them with multislice systems to produce images of equivalent quality.